The cyclins are a class of polypeptides which are involved in the control of the cell cycle. Three closely related human D-type cyclins have been identified, all of which interact with and activate cyclin dependent kinases (CDK) 4 and 6, although they have specialized function in distinct cell types. The cyclin D1 gene has been found to be overexpressed and/or deregulated by clonal chromosome rearrangements or by amplification in B cell lymphoma, in parathyroid adenoma, and in breast and squamous cell cancer. It has also recently been shown that cyclin D1 deficient mice have a defect in estrogen-mediated proliferation of breast epithelium during pregnancy (Sicinski et al, 1995, Cell 82; 621-630; Fantl et al, 1995, Genes & Development 9; 2364-2372).
Cyclin D1 is induced in response to mitogenic stimulation of quiescent cells and acts as an activator of CDK4 and CDK6. These cyclin D1/CDK complexes are key regulators of progression through the GI phase of the cell cycle and are involved in functional inactivation of the retinoblastoma family proteins (reviewed in (Beijersbergen and Bernards, 1996)). Cyclin D1 is amplified or over-expressed in a number of human malignancies, the most prominent being breast cancer, in which up to 50% of all cases have elevated levels of cyclin D1 (Buckley et al., 1993; Schuuring et al., 1992; van Diest et al., 1997). The relevance of cyclin D1 over-expression is underscored by the finding that tissue-specific transgenic expression of cyclin D1 in mice results in mammary hyperplasia and adenocarcinoma (Wang et al., 1994). Furthermore, cyclin D1 knockout mice show a marked defect in breast epithelium development during pregnancy and cyclin D1 reduces mitogen requirement of breast cancer cell lines (Fantl et al., 1995; Musgrove et al., 1994; Sicinski et al., 1995; Zwijsen et al., 1996). Cyclin D1 is over-expressed preferentially in ER-positive breast cancers, suggesting that cyclin D1 derives (part of) its oncogenic activity in breast cancer by acting on ER (Gillett et al., 1996; van Diest et al., 1997). We and others have recently made a connection between ER and cyclin D1 by showing that cyclin D1 can interact directly with the ligand binding domain of ER and can stimulate ER transactivation in a ligand-independent and CDK-independent fashion (Neuman et al., 1997; Zwijsen et al., 1997).
WO97/40378 also discloses that cyclin D1 interacts with the estrogen receptor (ER). Transcription is increased by formation of cyclin D1-ER complex which binds to the estrogen response element (ERE) found upstream of estrogen responsive genes. This finding provides a target for the control of cell proliferation, particularly in those cells which grow in response to stimulation by estrogen, e.g. breast tumour cells.
Several lines of evidence suggest that efficient transactivation requires additional positively acting factors termed coactivators (Pugh and Tjian, 1990). Several candidate steroid receptor coactivators (SRCs) have been identified. The first coactivator identified based on its ability to interact with the progesterone receptor was SRC-1 (Onate et al., 1995; Yao et al., 1996). This protein is the founding member of a family of related SRCs that include TIF-2/GRIP-1 (Hong et al., 1997; Voegel et al., 1996) and AIB-1/ACTR/RAC-3/p/CIP (Anzick et al., 1997; Chen et al., 1997; Li et al., 1997; Torchia et al., 1997). Several functional domains are highly conserved in all members of this family. For instance, the N-terminal regions contain basic helix-loop-helix (bHLH) and per-ARNT-Sim (PAS) domains. Both motifs are thought to be involved in protein-protein interactions and DNA-protein interactions (Yao et al., 1996). Interestingly, the bHLH-PAS domain is dispensable for SRC-1 activity, including receptor interaction and receptor activation (Onate et al., 1995; Yao et al., 1996). In addition, all SRCs contain multiple LXXLL (SEQ ID NO:6)motifs (L is leucine; X is any amino acid) in the central region of the protein. These motifs were recently shown to be involved in nuclear receptor interaction (Heery et al., 1997; Le Douarin et al., 1996; Torchia et al., 1997). Besides ER, SRC-1 also interacts with another coactivator of steroid receptors, CDP/p300, and both types of coactivators act synergistically to enhance ER transactivation (Chakravarti et al., 1996; Chen et al., 1997; Hanstein et al., 1996; Kamei et al., 1996; Smith et al., 1996; Yao et al., 1996). Both coactivators of the SRC-1 family and the p300/CBP family have intrinsic histone acetyl transferase (HAT) activity which is widely believed to be involved in chromatin remodeling during transcriptional activation (Jenster et al., 1997; Ogryzko et al., 1996; Spencer et al., 1997).